High-intensity discharge (HID) lamps are used as a light source which uses gas discharging principle to achieve illumination. As the HID lamp has a high luminance, large illumination area, and low power consumption, the HID lamp has been widely employed to illuminate a large open space or provide illumination for automobiles.
Referring to FIG. 1, the relationship of the lamp power versus the lamp voltage of the HID lamp according to the prior art is shown. As shown in FIG. 1, after the HID lamp ignited, the driving circuit for HID lamp outputs a constant current to heat the lamp electrodes. Thus, the lamp power and the lamp voltage of the HID lamp will rise gradually. When the lamp voltage rises continuously to exceed the switching voltage Vs under the constant power control mode, the lamp power outputted from the driving circuit for HID lamp will stop rising and maintain at a constant value. In the meantime, the lamp voltage will rise to a stable value gradually. Each time the HID lamp is ignited, the lamp voltage under the stable mode will rise continuously in accordance with the accumulated service time of the HID lamp. The rising of the lamp voltage under the stable mode will last until the burnout of the HID lamp. Generally, the driving circuit for HID lamp for conventional HID lamp (not shown) controls the lamp power directly through the lamp voltage and the lamp current. Nonetheless, in some applications the lamp voltage and the lamp current are difficult to be detected directly. Therefore, the control of the lamp power requires to be implemented in an indirect way. This would cause a problem of how to precisely control the lamp power of HID lamp.
The contemporary driving circuit for HID lamp accomplishes the lamp power control by providing a constant reference voltage. The principle of control is to compare a feedback signal generated by a feedback circuit with the constant reference voltage and generate an error signal in response to the comparison. The duty ratio or the switching frequency of the switch elements in the driving circuit for HID lamp is adjusted in accordance with the variations of the error signal, thereby maintaining the lamp power at a constant value. Due to the lamp's has negative impedance during its operation phase, the lamp voltage of the HID lamp is determined by the lamp current and the impedance of the HID lamp during the discharging operation as the HID lamp is operating under the constant power control mode. Therefore, the driving circuit for HID lamp is set to achieve constant power control by adjusting the lamp current.
Nevertheless, as the HID lamp has been used for a long haul and starts to age, the required lamp voltage to be applied to the lamp electrodes of the HID lamp will vary. Generally, the required lamp voltage to be applied to the lamp electrodes of the HID lamp is higher during the initial stage of the service term of the HID lamp. Therefore, if the aged HID lamp is driven by providing the required lamp voltage used in the initial stage of the service term of the HID lamp, the lamp power will be extraordinarily high and the lifetime of the HID lamp will be shortened.
Therefore, there is an urgent need to develop a driving circuit for HID lamp that can address the aforementioned problems.